The production of acetic acid by the carbonylation of methanol in the presence of an iridium catalyst and a promoter such as ruthenium is well known and is described, for example, in EP-A-0752406, EP-A-0849248, EP-A-0849249, and EP-A-1002785.
In operating carbonylation processes, such as the production of acetic acid, over extended periods of time, corrosion metal contaminants such as compounds of iron, nickel, molybdenum, chromium and the like form and build up in the carbonylation reaction composition. Such corrosion metal contaminants, if present in sufficient amounts may have an adverse effect on the carbonylation reaction.
U.S. Pat. No. 4,007,130 describes a process for regenerating a spent carbonylation catalyst solution comprising a rhodium or iridium catalyst by contacting it with a cation exchange resin in its hydrogen form.
European patent application, EP-A-0 265 140 describes contacting a catalyst solution comprising rhodium, lithium and corrosion metals with a cation exchange resin in its lithium form.
PCT patent application, WO 97/15544 describes a process for treating low water content carbonylation catalyst solutions which contain rhodium and an alkali metal to remove metallic corrosion products. The process comprises contacting the catalyst solution with an ion exchange resin and a sufficient amount of water to decrease the concentration of alkali metal ions to optimize removal of corrosion metal products.
Thus there remains the need for an improved process for removing corrosion metal contaminants from carbonylation catalyst solutions which comprise a carbonylation catalyst selected from iridium and/or rhodium, an alkali metal and/or alkaline earth metal and corrosion metals.
In a typical carbonylation process, such as in the manufacture of acetic acid or the manufacture of acetic anhydride, the carbonylation reaction composition from the reactor is passed to a flash zone for separation into a vapour fraction and a liquid fraction which comprises the majority of the catalyst components. This carbonylation catalyst solution is generally recycled back to the reactor. An iridium or an iridium/rhodium catalysed process is susceptible to small changes in alkali metal concentration and thus increases in the alkali metal concentration is likely to reduce the carbonylation rate and may cause a significant reduction in the rate. A rhodium catalysed process can tolerate greater concentrations of alkali metal however fluctuations in the alkali metal concentration will cause the carbonylation reaction rate to vary resulting in non-steady operation which is both undesirable and uneconomic. It has now been found that by partially loading a cation exchange resin with alkali/alkaline earth metal, the concentration of the alkali/alkaline earth metal in the carbonylation solution can be maintained thereby eliminating or at least mitigating the afore-mentioned disadvantages.